A plurality of data tracks are concentrically formed on a magnetic disk, such as a hard disk and a flexible disk. In a case where only reading or writing of information is performed with respect to the magnetic disk, the magnetic head is moved along the radial direction of the magnetic disk to make the head so that it corresponds to a specific data track (so-called seek operation), and then reading of information or writing of information is performed with respect to the specific data track. The positioning of the magnetic head for making the magnetic head so that it corresponds to the specific data track (so-called track following), as is well known, is performed by reading out the burst patterns recorded along the radial direction of the magnetic disk.
In order to make the aforementioned magnetic head so that it corresponds to a specific data track, Published Unexamined Japanese Patent Application No. 6-243617 has proposed a magnetic disk unit, where burst patterns consisting four kinds of burst patterns are recorded on the magnetic disk and the magnetic head is positioned by a position detection signal obtained by calculating four kinds of signals obtained from the burst patterns. In FIG. 8 there is shown the periphery of the (N-1)st to (N+2)nd data tracks as an example of these burst patterns.
As shown in FIG. 8(1), in the aforementioned magnetic disk unit, the burst pattern is constituted by a main burst pattern consisting of areas A and B and a slave burst pattern consisting of areas C and D. In this figure, arrow C direction shows a rotational direction of the magnetic disk and arrow D direction shows a radial direction. The main burst pattern is recorded on the areas A and B shifted in the radial direction of the magnetic disk by the width of the track. That is, the areas A of the width, which is substantially identical with the data track width, are arranged along the radial direction (arrow D direction), and also the same areas B as the areas A are shifted from the areas A in the radial direction (arrow D direction) by the track width and are arranged. With this arrangement, the main burst pattern where areas are arranged in zigzag fashion is formed. Also, by shifting the areas C shifted from the areas B by the half track width and the same areas D as the areas C in the radial direction by the track width and then arranging the areas C and D the slave burst pattern where areas are arranged in zigzag fashion is formed.
Now, if the burst patterns are read while moving the magnetic head in the radial direction of the magnetic disk, then a position detection signal MP which varies as shown by a solid line in FIG. 8(2) will be obtained for the main burst pattern. A signal (hereinafter referred to as a signal SB), obtained by reading the burst pattern regarding the areas B, is subtracted from a signal (hereinafter referred to as a signal SA) obtained by reading the burst pattern regarding the areas A. The resultant signal (SA-SB) is divided by the signal SB added to the signal SA. The resultant signal (SA-SB)/(SA+SB) is the position detection signal MP. Note that, in FIG. 8(2), the axis of abscissa represents the position of the magnetic head (physical position), more particularly represents the longitudinal center position of a gap formed in the magnetic head.
Also, a position detection signal NP where the phase is shifted, as shown by a solid line in FIG. 8(3), is obtained for the slave burst pattern. A signal (hereinafter referred to as a signal SD), obtained by reading the burst pattern regarding the areas D, is subtracted from a signal (hereinafter referred to as a signal SC) obtained by reading the burst pattern regarding the areas C. The resultant signal (SC-SD) is divided by the signal SD added to the signal SC. The resultant signal (SC-SD)/(SC+SD) is the position detection signal NP.
As shown in FIG. 8(2), the position detection signal regarding the main burst pattern linearly varies when passing through the vicinity of the widthwise (arrow D direction of FIG. 8) central portion of the Nth data track. This is also the same when passing through the widthwise central portions of other data tracks. Therefore, based on the level of the position detection signal the position of the magnetic head can be judged, and based on the level of the position signal the magnetic head can be positioned so that the central portion of the magnetic head (more specifically, the central portion of the read portion of the magnetic head, that is, the longitudinal central portion of a so-called gap) is located over the widthwise central portion of the data track.
However, in the position detection signal shown in FIG. 8(2), when the magnetic head is located in the vicinity of the boundary between adjacent data tracks, there is an interval where the level is substantially constant. This occurs because the longitudinal dimension of the read portion (gap) of the magnetic head is smaller than the width dimension of the data track. The aforementioned interval is called a dead zone, because the position of the magnetic head cannot be specified.
In the vicinity of the boundary of a data track such as this, as shown in FIG. 8(3), the position detection signal regarding the slave burst pattern linearly varies. Therefore, in the vicinity of the boundary of the data track, the position of the magnetic head can be judged based on the position detection signal regarding the slave burst pattern.
Therefore, by switching the position detection signal about the main or slave burst pattern in correspondence with the position of the magnetic head on the data track, a position detection signal which linearly varies over a wide range with respect to the movement of the magnetic head can be obtained. Based on the level of the signal, the position of the magnetic head can be judged. Based on the level of the position signal, the magnetic head can be positioned.
Incidentally, a magnetic head that reads out information by using a magnetoresistive element (hereinafter referred to as an MR element) has been proposed in recent years. The MR element is an element making use of an MR effect where, if a semiconductor is placed in a magnetic field, the advancing direction of the electrons or positive holes in the semiconductor will be varied by the magnetic field, the traveling path will become longer and the resistance value will increase. In a magnetic head which reads information by using this MR element and writes information by using a coil, a read gap and a write gap are separately provided and the longitudinal dimension of the read gap is made short to improve an error rate. In addition, the longitudinal center position (center) of the read gap and the center of the write gap are shifted from each other for reasons of the physical arrangement.
Therefore, in a case where writing of information is performed, it is necessary to locate the magnetic head in the position where the center of the write gap is aligned with the widthwise central portion of the data track, i.e., the position where the center of the read gap is shifted from the widthwise center portion of the data track. However, because the longitudinal direction of the read gap is shortened, as previously described, and the aforementioned interval which is a dead zone becomes long, the linear area where the position detection signal linearly varies with respect to the movement of the magnetic head is insufficient. Therefore, because the range where the position of the magnetic head can be detected becomes very narrow, it has been required to detect the position of the magnetic head with higher accuracy.
However, in the aforementioned magnetic disk unit, there are some cases where a size obtained as a position detection signal varies at every magnetic head or depending the radial direction of the magnetic disk, because of the state of the magnetic head, for example, a variation in voltages supplied to the magnetic head and in surrounding temperatures, an electrical offset contained in the burst pattern, and an offset produced at the time of magnetic recording.
That is, there are some cases where a position detection signal mp shown by a broken line in FIG. 8(2) and a position detection signal np shown by a broken line in FIG. 8(3) are shifted from a reference position detection signal (solid line) because of the state of the magnetic head or an electrical or magnetic offset. Therefore, the size of the position detection signal (amplitude, for example) cannot be obtained stably for all data tracks.
In this case, an ideal position detection signal which linearly varies as shown by a solid line in FIG. 8(4) cannot be obtained, and a separated characteristic is obtained as shown by a solid line in FIG. 8(4). In FIG. 8(4), the axis of ordinate represents the position of the magnetic head obtained from the position detection signal and the axis of abscissa represents the physical position of the magnetic head.
In order to eliminate the problems of the state of the magnetic head and the electrical or magnetic offset, it is conceivable to correct the gain of a servo loop used for a track following operation. However, because a difference in the gain of a servo circuit itself or in the gain of an actuator for moving the magnetic head (for example, a voice coil motor for rotating the magnetic head) must be added as a parameter, the position detection signal itself cannot be corrected. For this reason, the position detection signal becomes unstable.
Also, in order to obtain the position detection signal that linearly varies over a wide range like the aforementioned conventional technique, the switching of the position detection signal has to be performed at an optimum position. However, because a position detection signal that linearly varies cannot be obtained due to the state of the magnetic head and the electrical or magnetic offset, the position of the magnetic head is detected with the position detection signals mp and np departing from the straight portion (portion indicated by a broken line in FIG. 8(4) and so cannot be detected with accuracy. Therefore, the operation of the magnetic head becomes unstable.
A need exists for a magnetic head position detecting method and a magnetic disk apparatus which are capable of detecting the position of a magnetic head over a wide range by simple process that solve the above-identified problem. The present invention provides a solution to this and other problems, and offers other advantages over the prior art.